


Sincerity

by hyliank8



Category: Undertale (Video Game)
Genre: Ecto-Vagina (Undertale), Grey-Ace Sans (i think?), Implied past sexual coercion, Light Angst, M/M, Plot What Plot/Porn Without Plot, Praise Kink, Soul stuff, Vaginal Fingering, W. D. Gaster is not related to Skelebros, god i can't believe my own tags, i wasnt even trying to write porn i just want them to love each other holistically okay, mainly just soft tho, no dicks here my friends, not really soul sex but like. sex with soul bonuses???, past abusive relationship, there's literally no plot it's just gaster giving sans a good fucking
Language: English
Status: Completed
Published: 2019-12-12
Updated: 2019-12-12
Packaged: 2021-02-18 13:07:16
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,811
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21761353
Author URL: https://archiveofourown.org/users/hyliank8/pseuds/hyliank8
Summary: Gaster just wants to give Sans the wonderful treatment he deserves.
Relationships: W. D. Gaster/Sans
Comments: 15
Kudos: 72





	Sincerity

**Author's Note:**

> hi everyone it's k8 it's 2am i have a final tomorrow and i'm deep in sanster hell so naturally i wrote 2.8k words of skeleton porn to cope enjoy sans n gaster having a healthy relationship for once

It’s amazing, watching Sans come undone beneath him. Gaster drags his tongue over his lover’s lips and closes his mouth around the small bundle of nerves at the apex, gently massaging it with his tongue. Sans’ breaths are coming in short gasps, and Gaster can tell with satisfied certainty that Sans is feeling really, _really_ good—and it’s a nice change of pace, he reflects, seeing Sans be so authentically expressive with how he’s feeling.

Maybe they should do this more often.

“f-fuck—” Sans’ voice is little more than a whimper. Gaster’s gaze drags over his lover’s body, admiring the pristine ivory bones along the way, until it settles on his lover’s face. 

Sans’ eye sockets are slammed shut, and the gentle turquoise blush that had previously settled over his cheekbones has now deepened significantly. The flush has spread across his face, and its color has darkened to a navy hue.

It’s the most beautiful thing Gaster’s ever seen.

Gaster hooks his scarred hands underneath either of Sans’ ilia and shifts him closer; and he moves with ease, he’s so light. Bringing him closer gives Gaster an easier reach, and he utilizes his new advantage by pressing down a little harder with his tongue while still keeping his steady rhythm, working him with confidence.

Gaster continues watching his partner’s flushed face intently, idly stroking a thumb over his iliac crest. Sans’ teeth part ever-so-slightly and a little broken sound escapes, desperate and needy.

“g-gaster,” he gasps. “o-oh, gods, gaster—keep doing that pl-please, so good—”

Sans’ voice is scratchy and barely there and _stars_ , the sound is positively endearing. Gaster extends his magic to materialize a hand construct, and lovingly cups his small boyfriend’s face with it.

Gaster’s mouth releases his lover’s clit, and he brings one of his hands from under Sans’ ilium to rest atop his pubic symphysis, opting instead to use his thumb to lightly caress Sans’ sweet spot. Sans’ breath hitches again at the shift in sensation. His eyesockets crack open just enough for him to be able to peer down at Gaster. His eyelights are hazy, and it’s clear he’s completely lost in the pleasure.

“My stars, you’re beautiful,” Gaster murmurs, idly dismissing his conjured hand. “I think I quite like hearing you say my name like that.” He presses his thumb down a bit harder and revels in the needy sound that leaves Sans’ mouth. He’s being quite vocal—he seems to have a reaction for every little thing Gaster tries on him, and Gaster finds that he doesn’t mind that much.

Gaster returns his attention to his beloved’s pleasure. He brings his other hand from underneath Sans’ ilium to caress over his slit, moving his flattened hand up and down its full length. Sans is incredibly wet, Gaster notes with satisfaction; his fingers become soaked with his lover’s slick almost immediately. The thumb of his other hand is still working at Sans’ clit, and he feels Sans tense in anticipation under his ministrations. 

Gaster halts his hand’s movements along Sans’ slit to come to rest at his entrance and teases the opening with his forefinger, circling around it playfully. Sans whimpers, femurs trembling on either side of Gaster’s head.

Gaster looks up at Sans once more from between his legs. Sans’ eyes are squeezed shut again, and Gaster decides that it’s an adorable look on him.

“Sans,” Gaster says his name softly to get his attention. Sans cracks his eyes open immediately, very conscious of the plateau in stimulation. He certainly _seems_ desperate for it, Gaster notes, but he knows it's always worth it to check. Sans had been very trusting by opening up to Gaster about his past… “relationship,” and the fears that he now had to deal with as a result of it. Just the thought of it makes Gaster bristle, that someone could hurt Sans, could hurt someone as amazing as his best friend without any remorse for their actions. 

Sans’ eyelights are wavering, the buildup of magic in his body causing instability in their presentation. It’s such a vulnerable look—Gaster renews his vow that Sans will never have to feel anything but pleasure in a situation like this. No one will hurt Sans ever again; he’ll make sure of it.

“Is it okay if I continue?” Gaster is sure to look Sans in his hazy eyelights as he asks. He knows from experience that Sans will look away before answering if he’s unsure—he’s learned to not simply take a “yes” under all circumstances.

_(He remembers the first “yes” he got from Sans—the fearful tears threatening the corners of his lover’s eyes is a horrible image ingrained in his mind. Gaster thanks his stars that he noticed the tears, noticed the shakiness in his voice stemming from something other than lust before he unknowingly crossed that line.)_

Sans nods without breaking eye contact. “y-yeah,” his voice is still scratchy, and Gaster feels a genuine smile spread across his face at the simple indication of trust, feels the warmth of love for his partner blooming anew in his soul.

Gaster nods lightly. “Let me know if it’s too much, okay?”

Sans swallows. “o-okay.”

Gaster positions his forefinger once more and finally enters him, taking care to move slowly. Sans lets out a quick breath at the sensation, clearly loving the feeling of his lover filling him. He closes his eyes once more, focusing on the pleasant feeling, on the warmth of Gaster’s hands on him and inside of him.

Gaster starts to move. He goes slowly at first, careful not to hurt his sensitive partner. But as Sans’ tiny whimpers increase in both volume and intensity, Gaster takes it upon himself to heed his unspoken wishes and speed up his movements, working up a steady pace pushing in and out of him while the thumb of his other hand continues rubbing lightly over his clit. 

“You like this?” Gaster says, pleased by the little noises Sans is making. 

“y-yeah,” Sans manages. “it’s—fuck, it’s g-good.” 

A smile plays at the corners of Gaster’s mouth once more. “I’m glad,” he says. “You deserve to feel good, Sans.” 

Gaster rises from where he’d knelt on the floor at the edge of the bed and leans over his small lover’s trembling body, one hand abandoning its attention to Sans’ clit as he brings it up to cup his face again, while the other hand continues its movements driving into him. Gaster finds that his arm doesn’t have to stretch very far at all— Sans is so much smaller than him that he can still keep his elbow bent, his arm at a comfortable angle and still easily reach between his femurs with much room to spare. 

Gaster has always loved how much smaller Sans is than him—what had he done to deserve such an adorable, beautiful monster in his life, Gaster wonders idly. 

Noticing the shift in weight, Sans opens his eyes to find Gasters face inches from his own. His soul jumps in excitement in his rib cage, and he brings a hand forward, resting a hand on Gaster’s chest before clenching his fist into the soft fabric of Gaster’s shirt and pulling him closer into a searing kiss.

The kiss starts close-mouthed, but doesn’t stay as such for long. Sans coaxes his tongue construct into existence once more and allows Gaster to dominate his mouth. It’s overwhelming, Gaster’s tongue roughly stroking his own while his finger continues pumping in and out of him, his cunt pulsing with pleasure. 

Sans can feel his soul humming loudly under the onslaught of the intense stimulation, and feels a spark in his soul when Gaster lightly brushes Sans’ tongue with his teeth. Gaster seems to feel the spark from their close proximity and moans into the kiss, slipping the hand cupping Sans face behind Sans’ neck to bring him closer.

Gaster breaks off the kiss finally and moves down to Sans’ vertebrae, leaving wet kisses on the bone. And god, it’s so incredibly _hot_ , feeling Gaster’s lips, his _tongue_ on his neck, the hot breath on his bones; it sends shivers down his spine and intensifies the pleasure growing in his core. And his pleasure’s building fast—Gaster’s somehow managed to continue thrusting at a consistent pace through their kissing, and Sans feels his soul beginning to tremble from the pleasure.

Gaster always _was_ an excellent multi-tasker, Sans mirthfully reflects.

Gaster abandons Sans’ neck in favor of pressing their foreheads together and locking their eyes. The intense, loving expression on Gaster’s face overwhelms Sans with emotions— _he’s so sincere_ , Sans marvels. It's not something he's used to, someone caring about him so much, someone so concerned with how good he feels and—god, it strikes him again just how _much_ Gaster means to him. His soul is so warm being close to the person he loves most like this, and he knows it won’t be long until he comes if Gaster keeps going the way he is.

Gaster curls his finger upwards and Sans mewls, desperate and needy and so utterly _lost_ in sensation, his hand still buried in the front of Gaster’s shirt now clinging as though it’s the only thing left tethering him to reality.

“Is two okay?” Gaster murmurs into Sans’ ear, knowing he will understand what he means.

Sans nods enthusiastically. “y-yes, please,” he whines.

Gaster hums in satisfaction and slips in a second finger. Sans immediately groans at the new sensation, and Gaster knew it must take some adjustment to be stretched out as he is now— it's… rare, to say the least, that they take an intimate encounter this far. Which, in Gaster’s mind, simply makes it all the more special; Sans trusts him like he trusts no one else, and that warms Gaster’s soul in a way that nothing else does.

“f-fuck, ‘dings,” Sans gasps, barely able to speak as Gaster’s fingers pump in and out of him. “s-so good, it feels s-so amazing, y-you’re amazing—”

Gaster notices Sans’ walls begin to flutter around his fingers, and the fist not already caught in Gaster’s shirt clenches tightly, the sheets twisting in his grip. Gaster’s soul thrums excitedly—Sans is close.

“‘dings,” Sans gasps. “m-more please, m-more—“

Stars, his begging is amazing; Gaster _loves_ seeing his partner reduced to gasping pleads, loves that he’s the one who can give him so much pleasure. “So needy,” Gaster chuckles softly.

“f-fuck,” Sans says, so lost in pleasure he seems unconcerned with Gaster’s teasing. “c-can you—another, please—“ he’s blushing furiously, but lust has curtailed the influence of his self-consciousness.

Gaster’s soul flutters at the request, at the desperation creeping into his lover’s tone once more. Sans is so adorably into this—he isn't usually ready to escalate things so quickly, even this close to his peak. Gaster studies his lover’s flushed face intently, but dares not slow his thrusting--it takes a lot to get Sans worked up this much, to get him to drop his fears and his worries and every other inhibition constantly working against Sans’ ability to enjoy himself. Sans deserves this, Gaster knows—he deserves to be given everything he wants and more, deserves all the good feelings, all the pleasure in the world, and Gaster is more than enthusiastic at the idea of being able to provide that for him.

“Three?” Gaster clarifies, a protective part of him cautious of pushing any boundaries. 

“y-yes, please—” 

Gaster nods and gives Sans what he wants, slowly pushing in a third finger. Sans’s breath hitches at the new sensation, and then a quiet, hesitant moan escapes him. Humming in satisfaction, Gaster withdraws only to plunge back inside faster, reveling in the way his lover’s pussy stretches for him.

Sans squirms, little broken sounds falling from his mouth as Gaster works up a careful pace once more. Gaster calls two extra hands into existence to grab Sans’ ilia and pin his pelvis to the bed to keep his squirming from affecting Gaster’s pace. Sans whines at the dominant move, and a little possessive thrill runs down Gaster’s spine at the sound.

Gaster can feel that Sans is so incredibly close, feels the little tremors in his lover’s soul—and god, he’s so _proud_ of him. Sans _always_ struggles with allowing himself to be in vulnerable positions, but here he is—letting Gaster show him the good time he deserves.

“You’re doing so well, Sans,” Gaster murmurs. “I know this is a lot, but you’re doing wonderfully and I’m so proud of you… you’re close, aren’t you?”

Sans nods clumsily, drunk on the pleasure, his eyelights hazing at the praise. “y-yeah, i-i’m—d-dings, i’m gonna—you’re gonna make me—“ Sans voice increases in pitch until he breaks off with a whine, and Gaster feels his lover’s walls tighten.

“Mm, good boy…” Gaster whispers reassuringly. ”Come for me, hmm?” 

“hah- g- _gaster—!_ ” Sans’ spine arches off the bed as he cries out, his entire body shuddering as his orgasm crashes over him. Gaster can feel the intense waves of pleasure pounding through Sans' soul from a distance; he groans as he feels it pulse in his own soul, the secondhand pleasure overwhelming as it thrums through him. He’s careful to keep his pace, thrusting his lover through his orgasm despite the wonderfully distracting sensation echoing within the culmination of his being.

Gaster continues his thrusting until Sans gives an overstimulated whimper and pushes lightly against his chest. Gaster chuckles lightly and slides his hand out of his lover, bringing it to his mouth and sucking it clean. He moans softly, reveling in the fact that the taste is so uniquely _Sans_. 

Gaster wipes the remaining wetness off onto the sheets and stands up, dispelling his extra hands and carefully leaning over the edge of the bed to reach Sans’ face. He places a soft kiss on Sans’ cheekbone, and it’s adorable how much the sweet, simple gesture makes Sans blush.

“Feel good?” Gaster murmurs next to Sans’ ear canal.

“y-yeah,” Sans whispers, his voice still shuddery. “that was… really nice, ‘dings.”

Gaster smiles warmly. “I’m glad, my love. I love making you feel good.”

As Sans pauses to let his breathing even out, Gaster lightly brings his head down to rest his forehead on Sans’. His eyes close contently as he focuses on the sound of his lover’s soul slowly calming down, the intensity of its thrumming gradually fading away until it returns to its idle volume. He stays like that for a long moment, the only sounds in the room their mixed breaths and the idle humming of their souls. Gaster pulls away to look back into Sans’ eyes only when he feels Sans’ brow furrow.

“it’s… so weird,” Sans says, breaking the silence. “even after all this time being with you, it’s still so hard to get used to you—i dunno, actually caring about me, i guess?” He averts his eyelights, clearly insecure about his confession.

Gaster brow furrows as he considers this. After a moment, he cups the side of Sans’ face and delicately runs his thumb over his cheekbone. “I... understand, Sans. It’s okay. But I mean what I say; it makes me happy to see you happy. I hope one day accepting that will come naturally for you.”

Sans seems to contemplate this for a moment before replying. “yeah… yeah. me too.”

“But until then, we’ll work through it— _together_. And I’ll remind you as many times as I have to, okay?”

“...okay. thanks, ‘dings.”

“Of course, Sans. I love you.”

That gets Sans’ eyes to light up, and an easy smile spreads across his face. “yeah… i love you too, ‘dings.”

Gaster smiles. “Alright, you ought to sleep. I can’t imagine you’re not exhausted, hmm?”

Sans yawns, as if on cue.

Gaster chuckles lightly. “Yeah, that’s what I thought. Come on.” Gaster carefully gathers Sans up in his arms, cradling his lover close to his chest as he walks to the side of the bed that Sans often favors. Sans is near completely limp in Gaster’s arms, completely relaxed and almost boneless—certainly a feat for a monster of his kind.

Gaster positions him on the bed properly, lightly setting him down so that his head rests atop one of the fluffy pillows. He places a light kiss on Sans’ skull, earning an appreciative, albeit sleepy, hum from him before climbing into bed next to him.

Sans immediately cuddles himself close to Gaster’s chest, lounging an arm over his torso to hold him close as though it’s the most natural thing in the world to him. Gaster’s soul warms as he wraps an arm around Sans’ shoulders, returning the hold.

“Goodnight, Sans.”

“‘night, ‘dings.”

They drift off, the gentle sound of their souls humming in sync lulling them both into a content sleep.

**Author's Note:**

> ANYWAYS this was sitting in my drafts for months but it's finally posted!! hope you enjoyed!!! it's 2am and i'm going to SLEEP
> 
> come follow me on [tumblr](https://hyliantimelordin221b.tumblr.com/) if u want! i talk abt sanster a lot. i've also been posting on [twitter](https://twitter.com/K8Maybe) a bit recently.
> 
> leave me a comment and lemme know what you thought if you want!! pls!!! every time you leave a comment a sans is cuddled by his gaster.... think of the soft bois..... do it for them......


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